This disclosure is generally related to flexible antennas having materials that leak radio frequency (RF) energy and, in particular, to an antenna made of coax cable that efficiently leaks RF energy over a short length with low return loss
Leaky coaxial cables have been used over the past several decades for applications such as distributing signals inside tunnels and mines, and along roadways. See for example “Propagation Along a Coaxial Cable with a Helical Shield”, D. A. Hill, J. R. Wait, IEEE Transactions on Microwave Theory and Techniques, vol. 28, no. 2, pp. 84-89, February 1980 and “Theory and Analysis of Leaky Coaxial Cables with Periodic Slots”, J. H. Wang, K. K. Mei, IEEE Transactions on Antennas and Propagation, vol. 49, no. 12, pp. 1723-1733, December 2001. Typically, these are ordinary coaxial cables with slots cut into them, loose braids, or helical shields. They are often designed so that the signals leak out over a distance of several kilometers.
U.S. Pat. No. 5,936,203 to Henry Ryman describes a radiating coaxial cable comprising a center conductor, a dielectric core and a plurality of conductive strips wrapped around the dielectric in a spiral. In one embodiment two strips are continuous and wrapped in opposite directions around the dielectric core along the entire length. The present invention has a first wrap along the entire length but is distinguished from Ryman in that the second wrap is not continuous along the entire length.
HRL application Ser. No. 11/412,575 filed Apr. 26, 2006, incorporated by reference in its entirety, disclosed a new kind of leaky cable that is a few millimeters thick, and can radiate efficiently within a length of about one meter. However, high radiation efficiency was achieved at a price of high return loss. The other noticeable problem was significant variation observed in antenna gain over the desirable frequency ranges. HRL application Ser. No. 12/193,500 filed Aug. 18, 2008, incorporated be reference in its entirety, disclosed a leaky cable antenna with low insertion losses. This invention discloses a high radiation efficiency coaxial cable antenna with low return loss and improved antenna gain over the designed frequency ranges.
There is a need to provide coax cable that efficiently radiates RF energy in a short length with substantially uniform gain and low return loss.